Aqua

Aqua is one of the three main protagonists in Kingdom Hearts Birth By Sleep. She serves under Master Eraqus as a student and is the only one out of Ventus (still too young), Terra and herself to pass the Mark of Mastery exam. Aqua is like Ven's older sister to him, she is always taking care of him.

Throughout her jouney she pursues Terra, trying to bring him back. She is forced to face Vanitas on a few occasions and learns the truth of Ventus's existence. In the final battle, she and King Mickey fought Vanitas, who had possessed Ven, defeating him in the end.

After placing Ven in Castle Oblivion, she looks for Terra. Upon finding him in Radiant Garden, she discovers that he is being possessed by Master Xehanort, and he engages in a fight with her. Once defeated he falls into Darkness but Aqua jumps in after him. She enables him to leave but is left to stay in the Realm of Darkness. She later meets Ansem the Wise in the Dark Margin. He tells her of a boy who defends the light. A single tear falls from her eye as he says "Sora."

In the E3 2018 trailer for Kingdom Hearts III we get a new look at Aqua, who appears to have fallen under the influence of Master Xehanort. In the trailer she turns to the camera and we see her eyes have turned yellow just like his. She tells Mickey "it's too late", possibly meaning its too late to save her from the dark.